This invention relates to an infra-sonic alarm system for monitoring an enclosed area, such as the interior of an automobile, and providing an alarm signal when the integrity of the enclosed area is violated by an intruder.
The prior art is replete with detectors and alarms for monitoring enclosed areas including automobiles. Such detectors and alarms operate on various principles, including infrared and ultrasonics. These devices each have their advantages and disadvantages, which are well known to those working in the field of alarm or monitoring systems.
There exists U.S. Pat. No. 4,586,031 directed towards what is described as an infra-sonic detector and alarms using such a detector. The infra-sonic detector disclosed in this patent is disclosed as a microphone or other suitable detector for detecting air pressure variations, which is associated with filtering means for extracting the low frequency sounds of interest, which are below 20 Hz. The circuit and alarm arrangement of that patent includes a threshold circuit for comparing the signal from the low pass filter arrangement to a fixed reference to generate a trigger signal for activating an alarm. A manual sensitivity control is provided for adjusting the level of the fixed reference.
In the arrangement disclosed in U.S. Pat. No. 4,586,031, there is a description of various bands of frequencies, ranging from below 0.3 Hz to above 4.5 Hz. The patent contemplates providing sensing in these various bands of frequencies in order to avoid false alarms and the like. The patent states that: "The use of the B [0.3-0.7 Hz] and C [0.7-1.8 Hz] bands obviates any difficulties from very slow pressure variations (temperature rise of a vehicle situated in full sunlight, change from daytime to nighttime conditions, and so on) and fast variations (e.g. infra-sonics of the frequencies of bands D [1.8-4.5 Hz] and E [above 4.5 Hz] produced by a vehicle passing nearby)."